Police are close to a breakthrough in the former Russian spy poisoning case, according to UK Home Secretary Amber Rudd.

Ms Rudd yesterday revealed “more is now known about the substance” used against Colonel Sergei Skripal and his daughter.

But emergency services had to be called to an office in central Salisbury yesterday lunchtime following reports a person had been taken seriously ill.

Three fire engines and at least two ambulances arrived at premises close to Zizzi’s restaurant on Castle Street.

Police officers and paramedics were seen inside Sarum House, next door to the pizzeria.

Ms Rudd chaired a meeting of the UK government’s emergency committee in Whitehall to discuss the crisis as Mr Skripal (66) and his 33-year-old daughter Yulia continued to fight for their lives in hospital.

Ms Rudd said: “We need to keep a cool head and make sure we collect all the evidence we can. We need to make sure we respond not to rumour but to all the evidence and then we will need to decide what action to take.” She added: “We do know more about the substance.”

Scotland Yard counter-terror officers further extended cordons in the city overnight, while scientists at Porton Down, the UK’s military research facility, continue their investigation.

One theory being explored was that the substance could be the deadly nerve agent VX, which was used last year in the murder of North Korean dictator Kim Jung-un’s half-brother Kim Jong-nam.

Mr Skripal and Yulia were found unconscious shortly after 4pm on Sunday. They are thought to have been poisoned as they sat on a bench outside a shopping centre.

Detectives have urged anybody who visited Salisbury town centre on Sunday afternoon or who visited Zizzi’s – where the Skripals are believed to have eaten – or the Bishop’s Mill pub to come forward.

Russian President Vladimir Putin had vowed to kill Mr Skripal as he was being sent to the UK in a spy exchange in 2010. The emergence of Mr Putin’s warning will reinforce UK ministers’ belief that the attack was state-sponsored and perpetrated by the FSB Russian intelligence agency.